1. Field of the Invention
Embodiments of the present invention generally relate to thin-film formation, such as for solar cells, and methods for forming thin-films. More particularly, embodiments of the present invention relate to methods of forming an intrinsic-type silicon layer having mixed silicon phases formed in thin-film and crystalline solar cells.
2. Description of the Related Art
Photovoltaic (PV) devices or solar cells are devices which convert sunlight into direct current (DC) electrical power. Crystalline silicon solar cells and thin-film solar cells are two types of solar cells. Crystalline silicon solar cells typically use either mono-crystalline substrates (i.e., single-crystal substrates of pure silicon) or multi-crystalline silicon substrates (i.e., poly-crystalline or polysilicon). Additional film layers are deposited onto the silicon substrates to improve light capture, form the electrical circuits, and protect the devices. Thin-film solar cells use thin layers of materials deposited on suitable substrates to form one or more p-n junctions. Suitable substrates include glass, metal, and polymer substrates.
Typically, a thin-film solar cell includes active regions, or photoelectric conversion units, and a transparent conductive oxide (TCO) film disposed as a front electrode and/or as a backside electrode. The photoelectric conversion unit includes a p-type silicon layer, an n-type silicon layer, and an intrinsic-type (i-type) silicon layer sandwiched between the p-type and n-type silicon layers. Several types of silicon films, including microcrystalline silicon film (pc-Si), amorphous silicon film (a-Si), polycrystalline silicon film (poly-Si), and the like, may be utilized to form the p-type, n-type, and/or i-type layers of the photoelectric conversion unit. The backside electrode may contain one or more conductive layers. When the photoelectric conversion unit of the solar cell is exposed to sunlight (consisting of energy from photons), the sunlight is converted to electricity through the PV effect.
Thin-film solar cells are generally formed from numerous types of films, or layers, put together in many different ways. Most films used in such devices incorporate a semiconductor element that may comprise silicon, germanium, carbon, boron, phosphorous, nitrogen, oxygen, hydrogen, and the like. Characteristics of the different films include degrees of crystallinity, dopant type, dopant concentration, film refractive index, film extinction coefficient, film transparency, film absorption, conductivity, thickness, and roughness. Most of these films can be formed by use of a chemical vapor deposition process, which may include some degree of ionization or plasma formation.
Insufficient crystallinity of the silicon film for layers that should be microcrystalline or polycrystalline may cause incomplete formation and fraction of the film, thereby reducing the conversion efficiency in a PV solar cell. Additionally, conventional deposition processes of microcrystalline silicon film have slow deposition rates, which disadvantageously reduce manufacturing throughput and increase production costs.
To expand the economic use of solar cells, efficiency must be improved both in the solar cells themselves and in the processes used to manufacture solar cells. With the increase of energy cost as well as environmental concerns, there is a need for more efficient thin-film solar cells and more efficient formation of thin-films in solar cells.
Therefore, there is a need for an improved method for depositing a microcrystalline silicon film.